INSTREAM HABITAT USE BY BLUE DUCK (HYMENOLAIMUS-MALACORHYNCHOS) IN A NEW-ZEALAND RIVER

1. The feeding habitat of a river specialist, blue duck (Hymenolaimus malacorhynchos (Gmelin 1789): Anatidae), was characterized in terms of water depth and velocity on eight occasions over a 13-month period in a river in the central North Island of New Zealand using video to rec ord activity and relocate feeding sites. 2. Of the five feeding activi ties identified ('pecking', 'grazing', 'head-dipping', up-ending' and 'diving'), adult blue duck used mostly head-dipping (> 60% of feeding events on all dates), although diving or grazing from submerged surfac es of exposed boulders comprised major proportions of feeding behaviou r (up to 33%) on occasions. Variations in feeding behaviour between da tes partly reflected changes in antecedent flow conditions and the ann ual cycle of the birds. 3. Grazing and diving occurred in significantl y faster water (mostly 0.3-0.45 m s(-1)) and at significantly differen t depths (mean = 0.10 and 0.55 m, respectively) than head-dipping (0.2 0 m depth and 0.28 m s(-1) velocity). Adult feeding depths and velocit ies at four sites on different dates averaged 0.20 m and 0.31 m s(-1), respectively. Most feeding by 3-4-week-old ducklings occurred over a similar distribution of water velocities to adults but over a wider ra nge of depths. 4. Adult birds fed in significantly shallower and lower velocity water than was available on the two dates that comparisons c ould be made. Ducklings also fed over a slower range of water velociti es but were not selective in terms of water depth. 5. Energetically mo re expensive search methods were employed at times of high apparent en ergy demand to access flow microhabitats where larger bodied prey were more likely to be encountered. 6. These data indicate that, like othe r aquatic organisms, river birds can be influenced by basic hydraulic elements of river flow, but show at the same time that adult blue duck can accommodate variable lotic environments efficiently.